In recent years, radio tags have attracted attention as technique of identifying items of merchandise. Any radio tag has a memory that stores a tag ID, i.e., tag identifier specific to the radio tag. A radio tag may be attached to an item of merchandise, and a radio tag reading device may read the tag ID, thereby identifying the item of merchandise. The radio tags of this type are also known as radio-frequency identification (RFID) tags.
The radio tag reading device has an antenna. The device performs radio communication with radio tags that exist in a communication area where the radio signals transmitted from the antenna can travel. The radio tag reading device can therefore read the tag data from any radio tag located in the area, without contacting the radio tag. Radio-tag reading devices are classified into two types, i.e., stationary type and handy type. The stationary type has an antenna that is fixed in place. The handy type comprises a main unit and an antenna secured to the main unit and can be portable.
The radio tag reading device of handy type may be used, for example, in making an inventory. In this case, the items of merchandise are stored in shelves, each with a radio-tag attached to it. The person in charge of inventory holds the radio tag reading device and moves it toward each shelf holding items of merchandise subjected to inventory. Then, the radio tag reading device reads data from the radio tags attached to these items of merchandise held on the shelf. The data read from the radio tags is output from the radio tag reading device to a host apparatus such as a personal computer. From data read from the radio tags, the host apparatus identifies the items of merchandise to which the radio tags are attached.
If a radio tag reading device of handy type is used in making an inventory, however, the following problems will arise. The reading area of the radio tag reading device is within the communication area of an antenna. Therefore, if the radio signals transmitted from the antenna have a low output level, the area in which the radio signals can travel is small, and data may not be read from some of the radio tags existing in the area. Conversely, if the radio signals transmitted from the antenna have a high output level, the area in which the radio signals can travel is large, and data may unnecessarily be read from radio tags no data may be read from some tags existing outside the area.
Japanese Patent Application No. 2008-065386 discloses a radio tag reading device of portable type that has the function of determining a failure of recognizing radio tags. This device counts the tag data items read from radio tags via its antenna. The device further photographs the section in which to read data from radio tags extracts the images of all radio tags that exist in the section, and counts the images of radio tags. The device then compares the number of tag data items with the number of radio-tag images. If the numbers compared are equal to each other, the device determines that all radio tags existing in the section have been recognized.
The radio tag reading device disclosed in the above-identified publication needs to have a function of photographing the section in which to read data from radio tags and a function of processing video data in order to extract the images of all radio tags that exist in the section. Inevitably, the device is complex and can hardly be manufactured at low cost.